The proposed research project is aimed at understanding how interactions between afferent activity and central auditory neurons affects development of those neurons and their abilities to cope with changing activity levels. The first set of experiments will address how neurons in the cochlear nucleus respond to the onset and increasing levels of afferent input from the cochlear nerve during development with the implementation of mechanisms to regulate intracellular ion homeostasis. Previous studies have indicated that these mechanisms become prominent at about the same time as onset of afferent activity. The hypothesis that activity determines that implementation of these mechanisms will be experimentally evaluated in chick embryos by surgically or pharmacologically eliminating afferent activity to the nucleus during development. The homeostatic mechanisms will then be evaluated in cochlear nucleus neurons and compared to neurons from normal embryos. Protein quantification and dynamic optical imaging will be used for these evaluations. The ability of a neuron to regulate intracellular calcium is critical for its survival. Many proteins are involved in this regulation and combined to restrict not only the amplitude of a calcium signal, but also the time course and area of that signal. During development of the auditory system, the input that generates these calcium signals changes dramatically. The second set of proposed experiments aims to understand how this input affects the ability of cochlear nucleus neurons to regulate these changing calcium signals. Similar to the first set of experiments, surgical and pharmacological techniques will be used to eliminate afferent activity during development of the brain stem auditory nuclei, and protein quantification and dynamic optical imaging will be used to evaluate specific components and functions of the cellular calcium regulatory machinery. In addition, a specialized imaging technique will be used that allows great precision in measuring the temporal and spatial aspects of the calcium signals within neurons. These studies will contribute to understanding the interactions between afferent activity and the development of specialized central auditory neurons. This understanding will allow for more accurate evaluations of the impact of early sensory deprivation on central neurons. In addition, they will contribute to an animal model of congenital hearing deficits that is widely used for studies on the development and regeneration of auditory function.